"It's a big concern," Gruden, the former Super Bowl-winning coach for the Tampa Bay Buccaneers, told USA TODAY Sports. "Johnny has a lot of magic to him. There's not a more exciting college football player I've seen in the last few years."

Gruden, who worked with Favre as a Green Bay Packers assistant from 1992-94, says of the nine quarterback prospects he interviewed and worked out, Manziel stood out.

"I had more fun with Manziel than I did most guys," Gruden said. "I'd love to have him. It takes courage to pull the ball down and reverse field and do some of the crazy things that Favre and Manziel do. There's going to be consequences when sometimes it doesn't work out. But it takes a tremendous amount of guts and courage to go make a play when there's nothing there instead of throwing the ball away."

Gruden also compared Manziel to Hall of Fame quarterback Steve Young, who would improvise and run often early in his career. With some polish and direction, Gruden believes Manziel could also have a bright future in the NFL.

PHOTOS: Manziel's pro day

Former Texas A&M QB Johnny Manziel held his personal pro day at College Station, Texas, on March 27. Andrew Richardson, USA TODAY Sports

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"This kid (would have had) two years left in the NCAA," Gruden said. "So if you get him with a quarterback coach or anybody that aspires to be a quarterback coach and spend the time teaching this kid NFL defenses and your system, I can't imagine Johnny not being successful."

Senior Bowl executive director and Alabama football radio analyst Phil Savage doesn't think Manziel is in the No. 1 overall mix for the Texans.

Savage believes Houston is more likely to choose between the perceived best talent in this draft, South Carolina defensive end Jadeveon Clowney, or Central Florida quarterback Blake Bortles.

"After the way Clowney worked out at his pro day, they have more of a chance potentially to be able to trade out," Savage told USA TODAY Sports. "That's probably their No. 1 goal — to be able to slide down, making it a little more reasonable to be able to take Blake Bortles. That's the quarterback I think they like best.

Gruden believes Manziel is a dynamic playmaker who would fit O'Brien, and the former New England Patriots offensive coordinator would be the right coach to reign Manziel in as a more disciplined pocket passer.

Gruden called Manziel out on his Johnny Football quarterback image and believes Manziel is sincere about distancing himself from that bad-boy celebrity perception and will work hard to be great.

"Nobody is perfect, but you certainly don't want your quarterback flying around during the week. You don't want to read about your quarterback in the newspaper every day of the week," Gruden said. "I think he's learned a lesson, but he's got to prove that.

"I mean he ran for 1,400 yards in the SEC. He's got big hands and big feet and he's got more guts than most guys have."

Just one more evaluation for the Texans brain trust to chew on with the draft just three weeks away.

1. Jadeveon Clowney, DE, South Carolina – Mario Williams. Before the 2006 NFL draft, Williams faced many of the questions now being asked of Clowney. Williams' production during his final college season dropped off and he didn't seem to have as big of an impact on the game. Yet, his physical skills were never in question, which led him to become the No. 1 overall pick. USA TODAY Sports

2. Greg Robinson, OT, Auburn – Trent Williams. Like Williams at Oklahoma, Robinson was bulldozing run blocker for a dynamic running back at Auburn. Whichever team drafts Robinson will hope he can turn into the pass blocker Williams has become in the NFL. USA TODAY Sports

3. Jake Matthews, OT, Texas A&M – Joe Thomas. An offensive lineman's technique is as important as his physical ability. Thomas isn't the most athletic lineman, but he's by far the most consistent with his technique. Matthews is the most technically sound prospect in this year's draft class. USA TODAY Sports

4. Anthony Barr, LB UCLA – Aldon Smith.
Barr will be drafted on potential, and, like Smith coming out of Missouri, the UCLA Bruin has the athletic ability and physical profile (without the character questions that have dogged Smith in the NFL) to be a dominant edge rusher in a 3-4 defense. USA TODAY Sports

5. Khalil Mack, LB, Buffalo – Clay Matthews. The strengths of Mack's game begin with his versatility and extend to a ferocious attitude on the field. Like Matthews, Mack can be moved all over a defense to take advantage of his natural abilities. Their combine numbers were eerily similar, too. USA TODAY Sports

6. Sammy Watkins, WR, Clemson – Steve Smith. Watkins is a unique talent. There are not many receivers at 6-1 with return skills. Smith is four inches shorter than Watkins, but both can hurt a defense all over the field in a variety of ways and are excellent after the catch. USA TODAY Sports

7. Mike Evans, WR, Texas A&M – Vincent Jackson. Very few wide receivers have the size and ability to stretch a defense like Evans and Jackson can. The two are nearly identical in size (6-5, 230). They are also very physical wide receivers that use their bodies well to shield defenders, and to leap for catches. USA TODAY Sports

8. Justin Gilbert, CB, Oklahoma State – Patrick Peterson.
Gilbert has the size and speed to stick with opposing receivers in man coverage. Like Peterson, Gilbert’s body control allows him to make plays on the ball and create turnovers. USA TODAY Sports

9. Eric Ebron, TE, North Carolina – Jermichael Finley. In the ever-evolving NFL, tight ends are often oversized wide receivers that create mismatches against the linebackers and safeties. Ebron, like Finley, is a fluid route runner that can be lined up at multiple spots in an offense. Like Finley, he is not a strong blocker. USA TODAY Sports

10. Zack Martin, OT, Notre Dame – Logan Mankins. Martin will be drafted as a tackle, but his best fit in the NFL is at guard. Like Mankins, Martin will be a monster as a pulling guard leading runners through the hole in a power run system. USA TODAY Sports

11. Aaron Donald, DT, Pitt – Geno Atkins.
Very few defensive tackles can match the Donald’s production at the college level. At 6-0 and 285 pounds, Donald registered an astonishing 28.5 tackles for loss and 11 sacks. Like Donald, Atkins isn't the biggest defensive tackle, but he wins with quickness, strength and leverage. USA TODAY Sports

12. Darqueze Dennard, CB, Michigan State – Brandon Flowers. Dennard is a good cover corner who shouldn’t need much help over the top from a safety. Like Flowers with the Chiefs, Dennard will thrive in a system that allows him to play press man coverage. USA TODAY Sports

13. Taylor Lewan, OT, Michigan – Nate Solder. Lewan is highly athletic and impressed many with his workout at the combine. Lewan may not be a former tight end like Solder, but both are gangly linemen at nearly 6-8 with the type of length needed in a left tackle. Solder needed to work on his technique to become effective, just as Lewan will. USA TODAY Sports

14. Teddy Bridgewater, QB, Louisville – Matt Ryan. Bridgewater is a smart, accurate thrower who lacks some of the ideal physical tools teams look for in a franchise QB. Like Ryan, Bridgewater gets by with smart movement in the pocket and great field vision. USA TODAY Sports

15. C.J. Mosley, LB, Alabama – Daryl Washington. Both Mosley and Washington are 3-4 inside linebackers who are highly instinctive, athletic and quite comfortable dropping into coverage. The concern with both is that they're only 230 pounds. It hasn't affected Washington's career, but teams will question how Mosley holds up at the next level. USA TODAY Sports

16. Louis Nix III, NT, Notre Dame – Dan Williams. Nix is a large man who will provide a 3-4 defense with an ideal nose tackle. Williams demands a double team, which is vital for a Cardinals defense that uses inside linebackers as blitzers up the middle. Nix should draw the same type of attention in the trenches. USA TODAY Sports

17. Ha Ha Clinton-Dix, S Alabama – Eric Reid. Both Clinton-Dix and Reid are true free safeties with the ability to lock down the backend of a defense. Their final years on campus weren't as strong as their junior campaigns, but both were consistent throughout their careers playing against the run and pass. USA TODAY Sports

18. Timmy Jernigan, DT, Florida State - Jonathan Babineaux. Jernigan is an ideal fit as a three technique, attacking the gap between the guard and tackle. Like Babineaux for the Falcons, Jernigan should be a good player for a long time because of his explosion off the line, which makes him difficult to stop once he gets going. USA TODAY Sports

19. Kony Ealy, DE, Missouri – Michael Bennett. Missouri used Ealy in a similar fashion as the Seattle Seahawks do with Bennett. Both are defensive ends on run downs, but they are then moved to defensive tackle on passing downs. Both have quick first steps and very similar body types. USA TODAY Sports

20. Dee Ford, DE, Auburn – Chris Clemons
Like Clemons, Ford is a great pass rusher with the ability to play with his hand on the ground or standing up. Ford has a fairly well-developed arsenal of pass-rush moves but will have to prove he can be a three-down player at the next level. USA TODAY Sports

21. Blake Bortles, QB, UCF – Mark Sanchez.
While Sanchez, a former Top 5 pick, is generally considered a bust, the situations of these two quarterbacks are similar as they entered the league. Questions were raised about how ready they were. Neither was an advanced pocket passers in college. Both excelled when throwing on the move. And Bortles will likely be over-drafted due to his natural talents, just as Sanchez was. USA TODAY Sports

22. Johnny Manziel, QB, Texas A&M – Doug Flutie. Like Flutie, Manziel is known for making plays out of nothing despite his small stature. Unlike Flutie, Johnny Football is entering the NFL at a time where size is not as big of an issue as it was when Flutie broke into the league. USA TODAY Sports

23. Calvin Pryor, S, Louisville – T.J. Ward.
While Pryor is better in coverage than he's generally given credit for, he excels in setting the tone against the run. He’s an intimidating force across the middle. Ward is similar. Both explode into tackles and are at their best when playing downhill with the ball in front of them. USA TODAY Sports

24. Odell Beckham, WR, LSU – Golden Tate
Beckham is an undersized receiver but makes up for his slight build with toughness. He’ll provide a team with difficult catches on third down and good return skills – much like Tate did for the Seahawks the last few seasons. USA TODAY Sports

25. Marqise Lee, WR, USC – DeAndre Hopkins. Hopkins is bigger – and had a stronger final year in college -- but is a very similar athlete to Lee. Both are very good running after the catch, but Lee must prove he can stay healthy and be productive consistently. USA TODAY Sports